1. Field of the Invention
The present invention relates to a data transfer system for exchanging a quantity of data between two operation processing apparatuses.
2. Description of the Prior Art
Recently, full scale and complicated controls have become necessary with the development of production machines to huge sizes. In a machine tool system, a plurality of operation processing apparatuses such as a programmable sequence controller and a computerized numerical controller have been used for the executions of control functions and operation processings appropriate thereto, respectively. A need has been encountered for exchanging a quantity of control data between the processing apparatuses for the purpose of control intercommunication between the processing apparatuses.
The simplest data transfer method employed in the past is that in which a pair of input and output ports and a signal line are provided for transfer of a single signal between two operation processing apparatuses. However, such method necessitates the use of pairs of input and output ports and signal lines which correspond in number to signals to be transferred. Therefore, this method is not only difficult to practice in the instance where the number of signals to be transferred reaches more than several hundreds, but this method also disadvantageously increases the dangers of short-circuit and disconnection of the signal lines, thus resulting in the degradation of operational reliability.
These drawbacks may be obviated if a storage area, for data to be transferred, is established within a memory provided in each of the operation processing apparatuses. These apparatuses would alternately transfer the data stored in the storage area on a word (for example, 8-bits)-by-word basis. In order to execute such data transferring, however, each of the operation processing apparatuses must be given functions to write input data into the memory and to read out the data from the memory for delivery as well as a capability of identifying and processing, as input and output data, the data being stored in the memory. For this reason, where a programmable sequence controller, which has nothing but functions to test the operational states ("ON" or "OFF") of input elements in accordance with a sequence program stored in a memory and to energize or deenergize output elements in dependence upon the results of the tests, is used as the operation processing apparatus, such data transferring is not achieved. On the other hand, in the case of the operation processing apparatus composed of a general purpose digital computer, it is possible to execute the writing and reading-out of data into and from a memory of the computer. However, since the data processing according to a main control program or routine is interrupted at every occurrence of data receiving and data transmission, the more the quantity of data increases, the shorter becomes the time for the data processing according to the main control routine and this causes an inconvenience in control.
Further, there can be conceived another method in which one of two operation processing apparatuses, each composed of a general purpose digital computer, is provided with a common memory for storage of data to be exchanged therebetween and each of the processing apparatuses performs the writing and reading-out of data into and from the common memory. In this method, however, it may disadvantageously result that the two operation processing apparatuses simultaneously write data into the common memory or read out data therefrom, and in order to be prevented from such simultaneous accesses to the common memory, the two processing apparatuses must be operated in synchronous relation with each other or one of the processing apparatuses must be given preference over the other apparatus so as to cause the same to wait while it makes the access to the common memory. Additionally, since any one of the processing apparatuses which is remote from the common memory must be connected to the common memory through signal lines and photo-couplers, a considerable time is taken for the remote operation processing apparatus to execute the reading-out of data from the memory or the writing of data therein and therefore, reliable reading-out and writing-in of data cannot be carried out unless the operational cycle time of the remote operation processing apparatus is undesirably extended.